The Basics of Rotameters - Sensors

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he Basics of Rotameters - Sensors

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The Basics of Rotameters

You may have thought about using a rotameter to satisfy your flow measuring

requirements. But do you really know enough about these simple, cost-effective

devices to make the best decision? You will after you read this tutorial.

Oct 1, 2002

Sensors

Figure 1. The rotameter's operating principle is based on a float ofgiven density's establishing an equilibrium position where, with agiven flow rate, the upward force of the flowing fluid equals thedownward force of gravity. It does this, for example, by rising in thetapered tube with an increase in flow until the increased annulararea around it creates a new equilibrium position. By design, therotameter operates in accordance with formula for all variable-areameters, directly relating flow rate to area for flow.

Rotameters are the most widely used

type of variable-area (VA)

flowmeter. In these devices, the

falling and rising action of a float in

a tapered tube provides a measure of

flow rate (see Figure 1). Rotameters

are known as gravity-type

flowmeters because they are based

on the opposition between the

downward force of gravity and the

upward force of the flowing fluid.

When the flow is constant, the float

stays in one position that can be

related to the volumetric flow rate.

That position is indicated on a

graduated scale. Note that to keep

the full force of gravity in effect, thisdynamic balancing act requires a

vertical measuring tube.

Other forms of gravity-type VA

meters may incorporate a piston or

vane that responds to flow in a

manner similar to the float's

behavior. All these devices can be used to measure the flow rates of most liquids,

gases, and steam. There are also similar types that balance the fluid flow with a spring

rather than gravitational force. These do not require vertical mounting, but corrosive or

erosive fluids can damage the spring and lead to reduced accuracy.

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Typical rotameter types widely used in the process industries include two low-capacity meters often used aspurge meters (A). (Note the optional scale lengths); higher capacity, "standard" meters (B), shown here withfemale threaded connections (left) and screw-on pipe flanges (right); metal tube or armored rotameters forhigher flow capacities (C) and for low flows including purging (D).

The term rotameterderives from early versions of the floats, which had slots to help

stabilize and center them and which caused them to rotate. Today's floats take a variety

of shapes, including a spherical configuration used primarily in purgemeters (whichwill be discussed later on). The materials of construction include stainless steel, glass,

metal, and plastic.

The tapered tube's gradually increasing diameter provides a related increase in the

annular area around the float, and is designed in accordance with the basic equation for

volumetric flow rate:

(1)

where:

Q = volumetric flow rate, e.g., gallons per minute

k = a constant

A = annular area between the float and the tube

wall

g = force of gravity

h = pressure drop (head) across the float

With h being constant in a VA meter, we haveA as a direct function of flow rate Q.

Thus, the rotameter designer can determine the tube taper so that the height of the float

in the tube is a measure of flow rate.

Rotameter Design Components

The two basic components of every rotameter are the tapered metering tube and thefloat. Tube sizes vary from 1/16 to 4 in., with a

1/82 in. range being the most common.

Of course, each model has limitations as to capacity, temperature, pressure, and, in the

case of liquids, viscosity.

Glass Tube Rotameters. With a tapered metering tube made of borosilicate glass,

this was the original rotameter. Introduced in the mid-1940s, it is referred to as a

"general-purpose" rotameter. Because the float is normally visible in the tube, the

meter shows flow rate readings directly

on scale graduations on the glass

surface. Low-capacity glass tube meters

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Rotameters and the Market

For many years, rotameters were quite popularbecause they provided a local readout and asimple means of flow control for a wide variety ofgases and nonviscous liquids. And they wererelatively inexpensive. Then newer electronic andmore complex flowmeter designs began enteringthe marketplace with claims of superiority, albeitfor a price. Ultrasonic, magnetic, vortex shedding,

and smart dp transmitters competed for thespotlight. Yet today there are some 40 rotametersuppliers listed in current references and you haveto wonder how they all survive if their productshave been supplanted by these newertechnologies. But rotameters are still finding manyapplications in the process industries, as well as inresearch and development laboratories. In fact,worldwide sales of rotameters are estimated atabout $250 million a year. Some of the reasonsfor their continuing popularity are set out in thesidebar "Key Advantages of the Rotameter."

are used extensively in purge systems,

where they are calledpurgemeters. The

meter on the far left has a 3 in. scale

length, a needle valve on the inlet, and a

constant-flow differential pressure

regulator mounted on it. The meter next

to it has a 5 in. scale and no regulator.

Larger capacity meters can be adapted

to measure a variety of liquids and

gases and are still used except where

glass is prohibited for safety reasons.The meter third from the left has

screwed end fittings; the one on the

right, flanged ends.

Glass tube rotameters are typically used

for simple but reliable indication of

flow rate with a high level of

repeatability. Alarm contacts can be

easily added to provide high-, and/or

low-flow signals, in which the contact

is activated as the flow rate either drops

below or rises above the set point.

Linear scale graduations can be an arbitrary 0%100% for the meter range. Calibration

can be direct reading in terms of a specific gas or liquid, or a graph that plots meter

readings vs. flow rates in terms of the fluid being measured. Such graphs make it easy

to adapt a meter to handle fluids other than those for which it was bought; changeover

is simply a matter of having a different conversion chart designed for the new fluid.

The meter assembly's metal body is rigidly constructed to maintain tube alignment. The

various types of end fittings provide process pipe connections, either threaded female

or flanged. O-rings or packing glands at either end of the tube seal it to the end fittings.

Some designs provide for easy removal of the glass tube for cleaning or range change

without pulling the meter from the pipeline.

Metal Tube Rotameters. These devices, also known as armoredmeters, are designed

for applications where the temperature or pressure exceeds the limits of glass tubes.

Flow rate is indicated by a pointer on an indicating scale by means of a magnet inside

the float, magnetically linked to the pointer. Designed for indication only, metal tube

metersrequire no external source of electric power. They may also be specified in

applications requiring remote transmission of the measured flow rate, a feature not

generally available with glass tube meters.

Available in a variety of tube and float combinations, metal tube meters are generally

made of corrosion-resistant type 316 stainless steel. They are well suited to measuring

steam flow where conditions or regulations prevent the use of glass, and useful as well

where the nature of the fluid would preclude reading a float position. The meter shownhere is constructed of all 316-Ti stainless steel. The equally corrosion-resistant float is

magnetically coupled to the scale pointer and can also be coupled to high- and/or low-

flow alarms or to an electronic converter that generates a 420 mA signal for

transmission over a 2-wire cable to a remote indicator, recorder, or controller.

The signal can also provide a local digital display, HART protocol output, and scaled

pulse output for flow totalization. The HART output permits remote communication

over the same 2-wire cable that carries the flow rate signal. This provides remote

digital access to the meter and a link to plantwide control systems.

A low-capacity design is available for low flow measurements at high pressure and



4/12


temperature, or where the use of glass is restricted for safety reasons. Its integral needle

valve and constant-flow, differential pressure (dp) regulator are useful when it is used

as a purgemeter.

This small armored meter can also incorporate contacts for maximum and/or minimum

alarms and a 420 mA analog output. Their NPT process connections are available in1/8,

3/8,1/2, and 1 in. sizes.

Plastic Tube Rotameters. Plastic tube rotameters can be an entirely suitable, very

cost-effective alternative to glass or metal meters for a wide variety of fluidmeasurements. One popular model is made of a single piece of clear acrylic that is

practically unbreakable in most industrial process applications. Often used as a

purgemeter, this type is a low-cost, reliable solution for many OEM applications.

Float Designs. The float in small purgemeters is usually a ball made of black glass,

stainless steel, sapphire, Carboloy, or tantalum. For larger sizes in both glass and metal

tubes the float is generally machined from corrosion-resistant materials with variations

to suit the application. Floats are available in a variety of shapes and materials, with

varying densities that can be used to change the meter's range and to resist corrosion

from the measured fluid. While Type 316 stainless steel is common, floats are also

available in tantalum, Hastelloy C, Monel, Teflon, and PVC.

Use in an Orifice Bypass

To cost-effectively handle larger flow rates of liquids or gases, a rotameter can be

installed in a bypass line around an orifice plate. The differential pressure produced by

the orifice causes a relatively low flow through the rotameter that can be a measure of

flow through the main pipeline. Key benefits include:

q Usefulness where the measurement must be made in a hazardous or remote area,

or where electric power is either not available or would be potentially dangerous

q Rangeability that can be 121/2:1, as compared to 4:1 for dp meters

q Scale readings that can be graduated in direct units for flow in the main pipeline

q Changing the range or cleaning the tube without disassembling the meter or

removing it from the bypass line.

PurgemetersA Major Class of Rotameters

In terms of total worldwide rotameter sales, the models classified as purgemeters

dominate the field today. According to the ISA, a purgemeter is "designed to measure

small flow rates of liquids and gases used for purging measurement piping." As can be

seen from the five application examples below, however, purgemeters are sometimes

used for other tasks. All purgemeters have one important characteristic in common:

they facilitate setting and accurately controlling the low flow rates involved. For water,

the rate is typically well under 1 gpm; for air, it is


5/12


Five Sample Applications

Bubble Tube Purge for Liquid Level Measurement. Bubble tubes are a common

and inexpensive way to measure liquid level by measuring hydrostatic head, which is

density dependent. Figure 2 shows systems designed for open vessels and for closed

vessels under pressure. Each requires only one purgemeter with a pressure regulator.

Figure 2. Bubble tubes determine liquid level by measuring the hydrosstatic head, which isdensity dependent. The principle works for both open and pressurized closed vessels.

In an open vessel, at very modest cost, a simple indicating pressure gauge can measure

the backpressure from the dip tube. Another technique is to use a pressure recorder,

with or without control. Yet another is the widely used dp transmitter, with back

pressure connected to the high side and the low side left open to atmosphere.

A pressurized vessel requires two dip tubes with one above the maximum level,

exposed to the vessel pressure and connected to the low side of a dp transmitter. The

other dip tube, extending down to near the bottom of the vessel, thus can be a measure

of head due to liquid level. The effect of operating pressure that may vary is eliminated.

The purgemeter is commonly used in this application, with certain precautions that

apply to density measurement such as keeping the flow rate low enough to ensure thereis no incorrect increase in back pressure. The air or gas supply pressure, however, must

exceed the maximum line pressure by ~10 psi.

Purging Pressure Taps of Orifice Flowmeters. A very common method of

measuring flow rate requires the creation of a differential pressure across an orifice

plate installed in the flow line (see Figure 3).

Figure 3. Constant-flow purgemeters prevent a pressure-sensing device from fouling orplugging.



6/12


A dp transmitter is generally used to measure the pressure differential by means of

pressure tap lines connected upstream and downstream from the orifice. The supply

pressure should exceed line pressure by ~10 psig.

If the measured liquid or gas is the least bit corrosive or dirty, or contains solids, the

system designer should use a gas or clean liquid to purge the pressure tap lines. To

accomplish this, a pair of purgemeters, often with pressure regulators, feeds the purge

fluid into the pressure tap lines. When an orifice meter is to measure gas flow, the tap

is installed above the orifice. The purge fluid is usually air or an inert gas such as

nitrogen.

For liquid flow, the taps are positioned below the orifice and the purge fluid is

generally water. For heavier petroleum products, the purge may be kerosene or some

other clean, light hydrocarbon.

Continuous Forced Lubrication of Bearings. The bearings on heavy, rotating

equipment require continuous, forced lubrication to extend their functional life and

reduce machine downtime. A purge-type flowmeter (see Figure 4) can be used to set

the flow rate of lubricant as it is pumped to the bearings.

Figure 4. Expensive equipment can be made fail safe by adding an alarm to the rotameterthat monitors lubricant flow.

An electrical alarm is often added to the meter to alert the operator should the flow rate

fall too low. An electrical alarm is a positive, fail-safe device in that it measures flow

directly rather than inferentially. For this application, the rotameter should be

calibrated for the higher viscosity of the lubricating oil.

Bubble Tube Purge for Density Measurement. The density of a liquid is defined as

mass per unit volume (e.g., grams per cubic centimeter or pounds per cubic foot) at a

stated temperature. One of the oldest and simplest ways to measure density is by

measuring the hydrostatic head in a tank (see Figure 5).



7/12


Figure 5. One type of continuous liquid density measurement system incorporates two diptubes, a constant-flow purgementer, and a dp transmitter.

Two bubble tubes are immersed in a tank at different depths. The differential back

pressure of the purge gas in the two tubes, P, is a measure of the head, which in turn

is a function of liquid density, , and the difference between the depths of the two tubes

(L1- L2):

(2)

A pair of purgemeters, typically equipped with pressure regulators, can ensure a small,

uninterrupted flow of air or inert gas into the bubble tubes. The flow rate must be low

enough that pressure drop through the purge piping does not increase head back

pressure. The pressure differential can be measured with a dp transmitter, enabling

remote transmission and automatic control.

Use of Purgemeters with Gas Analyzers. Many types of gas analyzers require a

controlled, small rate of sample flow into the analyzer. Some analyzers also require the

introduction of modifying gases with the sample. Since the flow rates involved areusually very low, purgemeters are often an integral part of the analyzer cabinet.

Some Advice on Choosing a Rotameter

Assume you believe a rotameter may be a good solution to your flowmetering problem.

The benefits it offers are appealing and a check of the guidelines in the sidebar "Guide

to Deciding on a Rotameter" and Table 1 indicates

The Viscosity Immunity Ceiling

Depending on the tube and float combination for

a given rotameter measuring a specific liquid, theflow rate reading can be affected by viscosityabove a certain operating value. This isdetermined by a factor known as the viscosityimmunity ceiling, which relates to the actualviscosity in centipoises and the liquid's operatingdensity. You should therefore give the supplier thenecessary values so that this factor can be takeninto account as necessary.

that your application is in the ballpark

in terms of flow rates, operating

pressure and temperature, accuracy, and

the fluid you want to meter. Your best

bet is to collect certain pertinentinformation and take it to a rotameter

manufacturer or distributer, who can

help you select the correct meter for

your application. The data you should

have in hand include:

q Fluid type. The more

information on the nature of the

fluid, the better. Is it liquid, gas, or steam? If it is highly corrosive, give its

chemical name, e.g., sulfuric acid, and its concentration. Are there entrained

solids? And so on.



8/12


q Fluid density. Give specific units, such as grams per cubic centimeter, at a

specified operating temperature. Alternatively, you can specifyfluid-specific

gravity, which is the ratio of the fluid density to the density of water (in the case

of a liquid) or to that of air (in the case of a gas), at a specified temperature.

q Fluid viscosity. Again, use specific units such as centipoises or centistokes at a

specified operating temperature. (See the text box, "The Viscosity Immunity

Ceiling.")

q Operating and maximum temperature and pressure. This information is a

mustfor gas applications.

q Flow rates. What are the minimum and maximum?

q Required functions. What tasks will your system be expected to do? Will youneed to incorporate indicators, control devices, alarms, or remote transmission

capabilities?

q Indicating scale type. Do you want your readout in percent, direct reading, or

some other format?

q Materials of construction. Don't forget system components such as end

fittings, O-rings, regulators, and valves.

q Valve requirements. Will you need, for example, a needle valve on the flow

inlet or outlet?

Screen 1. The counter/display logic all fits inside a single user-configurable cPLD. Programmability allows designers tospecify the logic they want, instead of merely settling for what is available.

Sizing a Rotameter

Most manufacturers have taken all the factors and data involved in sizing a rotameter

and developed a sophisticated software program available on one CD for use on a PC.

One such program can be downloaded from ABB's Web site (see Screen 1).

These programs include volumes of pertinent data that will guide you through the

sizing process. For example, assume you need to measure methane gas at a rate of 300

scfm. You can access a reference table that lists the properties of all major gases. You

enter the necessary sizing data for methane such as operating pressure and temperature,

maximum flow rate, and meter type such as glass or metal tube.

The program then suggests that to meter methane under the conditions indicated, a 2 in.

meter with a specified type of glass tube and float will do the job and the flow range

will be 25335 scfm. If you prefer a metal tube, all you need do is click on Metal Type

on the screen. This new tool does away with having to convert the actual maximum

flow rate to a standard rate based on the water or air equivalent, along with density and

viscosity variables and their requisite equations.

http://www.abbinfozone.com/http://www.abbinfozone.com/


9/12


Summary

This article has addressed the operating principle and advantages of the rotameter,

along with some basic information about their applications. In addition to being the

most economical, these devices are still the most practical solution to many flow

measurement problems.

For Further Reading

Dictionary of Measurement and Control: Guidelines for Quality, Safety, &

Productivity. 1995. 3rd Ed., Instrument Society of America.

Dolenc, John W. Jan. 1996. "Choosing the Right Flow Meter." Chemical

Engineering:22-32.

Handbook, Process Measurement and Analysis. 1999. 3rd Ed., Bela G. Liptak, ed.,

CRC Press.

"Rotameters/Variable-Area Flowmeters." Sept. 1996.Measurements & Control.

Spitzer, David W. 1990. "Variable Area Flowmeters,"Industrial Flow Measurement,

ISA Resources for Measurement and Control Series.

SIDEBARS:

Guide to Deciding on a Rotameter

The information here is necessarily generalized, so consult the suppliers if your specific applicationappears to favor the use of a rotameter.

Service. Measuring volumetric flow rates of many liquids and gases, including steam (see Table 1

for certain limitations)

Flow range. Measuring liquid (water) flow rates from 0.65 cc/min. to 530 gpm in pipe from1/8 to 4

in. dia. and gases (air) from 47 sccm to 860 scfmDesign pressure (typical). Metal tube, to 1500 psig; glass tube, to 300 psig; plastic tube, to 100

psigDesign temperature (typical). Metal tube, to 900F; glass tube, to 250F; plastic tube, to 150

FScale. Linear

Signal. Visual and/or electronic

Accuracy. Typical accuracy 2% to 10% F.S., depending on type, size, and calibration

Repeatability. 0.5% to 1% F.S.

Viscosity. Liquids up to 200 cP

Rangeability. 5:112:1

Sizes. Up to 4 in.; also used as a bypass meter around a main line orifice for larger pipe sizes

End Connections. Flanged or threaded

TABLE 1

Selection for Fluid Duty

Nature of Fluid Suitability*

Liquid Clear Yes

Dirty Yes

Slurry No

High viscosity ?

Low viscosity Yes

Corrosive Yes

Gas Low pressure Yes

High pressure ?



10/12


Steam Yes

Service Conditions Reverse flow No

Pulsating flow No

*Suitability QualifiedYes=Gnerally suitableNo= Not suitable as a rule?=Worth considering

Key Advantages of the Rotameter

Easily maintained readability. With a glass tube meter, the principal metering elements (floatand tube) as well as the fluid being metered are clearly visible. You can immediately see anyaccumulation of dirt or other foreign matter deposited on the float or tube walls. A glass tube rotameter,kept clean, can therefore be used as a backup to check the performance of some other type of remote-reading flowmeter.

Sustained high repeatability. The float moves freely in the metering tube, without friction or

hysteresis. The rotameter thus attains the ideal design goal of having high repeatability andmaintaining it over years of service.

Wide rangeability. Rotameters offer a wide range of flow measurement, referred to as

rangeability. A ratio of 10:1 from maximum to minimum flow rate is typical. This means that a minimum

flow rate as low as 1/10 of the maximum flow rate can be measured without impairing the repeatability.

Linear scale. Because area variation is the measure of flow rate (rather than head or differential

pressure), the calibration curve is practically a straight line. This means the meter can have anindicating scale whose divisions are essentially spaced evenly (i.e., a linear scale). Compare this forreadability with any differential pressure flowmeter. Rotameter scale readings are not compressed(and harder to read) at low flow rates. You can therefore read flow rate with the same degree ofaccuracy throughout the entire range.

Low pressure loss. Because the area between the float and tapered tube increases with flow

rate, pressure loss (pressure drop) across the float is low and relatively constant. This reducespumping costs. Furthermore, a meter can be selected to provide a lower drop by using an oversizetube with a light float.

Compensation for viscosity changes. The float can be designed to compensatefor normal

variations in viscosity and density so that certain viscous oils and chemicals, such as sulfuric acid, canbe measured accurately in spite of wide temperature changes.

Increasible readability. The float travels vertically some 5 in. and can be as long as 24 in. The

meter designer can shorten the flow range and lengthen the float travel to increase measurementaccuracy to a degree unattainable with any other flow rate measuring instrument.

Readily corrosion-proofed. Because of its design simplicity, a rotameter can be economically

constructed of highly corrosion-resistant materials. It can therefore measure fluid flows that no othertype of meter will handle with continued success.

Easy to install and maintain. The inherent simplicity of design makes the rotameter easy toinstall and maintain. It mounts vertically in the pipe without pipe taps, connecting lines, seal pots, orvalves, or requirements for a straight run of pipe upstream or downstream as is necessary with a dptransmitter, nor is there a need to keep such parts free of foreign matter.

Can serve as a sight glass. A glass tube rotameter installed in a process line can also serve as

a sight glass to show that the process fluid is flowing in the line.

Needs no electric power. The simple indication of flow rate locally requires no connection to an

electric power source, and hence make explosionproofing unnecessary where flammable fluids maybe present.

Can measure very low flow rates. Liquid flow rates down to 1 mlpm and equally low gas flow



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rates can be measured.

Easily converted to measure different fluids. As described under "Sizing a Rotameter", a

model installed for service on one fluid can be recalibrated to measure another, taking into account itsspecific characteristics. Today, a CD software program makes this a quick and easy task.

A familiar flowmeter. Many prospective users understand or readily pick up the simple operation

of a rotameter Where the instrument can do the job, this fact should be a primary selection factor. Whypick a more complex and expensive measuring method with possible application and maintenanceproblems?

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Date post:	07-Apr-2018
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The Basics of Rotameters - Sensors

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